Fluid dispensers are well known for dispensing hand cleaning fluids as may be provided, for example, in washrooms and hospitals. Examples of such dispensers include those disclosed in U.S. Patent Publication US 2008/0121663 to Ophardt et al, published May 29, 2008; U.S. Patent Publication US 2010/0288788 to Ophardt published Nov. 18, 2010; U.S. Patent Publication US 2011/0017769 to Ophardt published Jan. 27, 2011; U.S. Patent Publication US 2007/0158363 to Ophardt published Jul. 12, 2007 and U.S. Patent Publication US 2010/0147879 to Ophardt et al published Jun. 17, 2010, the disclosures of which are incorporated herein by reference. With such dispensers, as fluid in the reservoir is used up, the fluid needs to be replaced as by refilling a refillable reservoir or removing and replacing an empty reservoir with a reservoir which is filled with fluid.
U.S. Pat. No. 9,027,788 issued May 12, 2015 to Ophardt et al, the disclosure of which is incorporated herein by reference, teaches an optical fluid level gauge for determining the height of a fluid in a bottle. The applicant has appreciated that the optical fluid level gauge taught by U.S. Pat. No. 9,027,788 suffers from a number of disadvantages. While the optical fluid level gauge taught by U.S. Pat. No. 9,027,788 is advantageous for use with collapsible bottles, its ability to accurately predict fluid levels in non-collapsible bottles is not adequate to meet requirements for real time monitoring of fluid levels, particular where monitoring is desired for relatively accurate estimation of the actual level of fluid remaining in the reservoir at any time and/or the changes of fluid levels with time.